Java高并发网络编程(四)Netty

在网络应用开发的过程中，直接使用JDK提供的NIO的API，比较繁琐，而且想要进行性能提升，还需要结合多线程技术。

由于网络编程本身的复杂性，以及JDK API开发的使用难度较高，所以在开源社区中，涌现出来了很多对JDK NIO进行封装、增强的网络编程框架，比如Netty、Mina等。

一、Netty简介

https://netty.io/ 官网

Netty是一个高性能、高可扩展性的异步事件驱动的网络应用程序框架，它极大简化了TCP和UDP客户端和服务器开发等网络编程。

Netty重要的四个内容：

• Reactor线程模型：一种高性能的多线程程序设计思路
• Netty中自己定义的Channel概念：增强版的通道概念
• ChannelPipeline职责链设计模式：事件处理机制
• 内存管理：增强的ByteBuf缓冲区

整体结构图

二、Netty线程模型

 为了让NIO处理更好的利用多线程特性，Netty实现了Reactor线程模型。

Reactor模型中有四个核心概念：

• Resources资源（请求/任务）
• Synchronous Event Demultiplexer同步事件复用器
• Dispatcher分配器
• Request Handler请求处理器

 Netty启动时会构建多个Reactor

EventLoopGroup初始化过程

 

 

两组EventLoopGroup(Main&Sub)处理不同通道不同的事件

public final class EchoServer {
static final int PORT = Integer.parseInt(System.getProperty("port", "8007"));

    public static void main(String[] args) throws Exception {
     // Configure the server.
        // 创建EventLoopGroup accept线程组 NioEventLoop
        
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        // 创建EventLoopGroup I/O线程组
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        final EchoServerHandler serverHandler = new EchoServerHandler();
        try {
            // 服务端启动引导工具类
            ServerBootstrap b = new ServerBootstrap();
            // 配置服务端处理的reactor线程组以及服务端的其他配置
            b.group(bossGroup, workerGroup2)
             .channel(NioServerSocketChannel.class)
             .option(ChannelOption.SO_BACKLOG, 100)
             .handler(new LoggingHandler(LogLevel.DEBUG))
             .childHandler((ChannelInitializer)(ch)-> {
              ChannelPipeline p = ch.pipeline();
              p.addLast(new EchoServerHandler);
             });

            // 通过bind启动服务器
            ChannelFuture f = b.bind(PORT).sync();

            // 阻塞主线程，直到网络服务被关闭
            f.channel().closeFuture().sync();
        } finally {
            // 关闭线程组
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

 

EventLoopGroup bossGroup = new NioEventLoopGroup(1);

追踪NioEventLoopGroup源码，会发现是创造很多NioEventLoop

public class NioEventLoopGroup extends MultithreadEventLoopGroup {
    ...
    public NioEventLoopGroup(int nThreads) {
            this(nThreads, (Executor) null);
     }
    ...

追踪到父类

public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {

    private static final InternalLogger logger = InternalLoggerFactory.getInstance(MultithreadEventLoopGroup.class);

    private static final int DEFAULT_EVENT_LOOP_THREADS;

    static {
        DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
                "io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));

        if (logger.isDebugEnabled()) {
            logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
        }
    }

    /**
     * @see MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, Executor, Object...)
     */
    protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
        super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args); // 默认是cpu核数*2
    }
...

追踪父类

public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {
// 代码省略
    
    // 多线程的事件执行器
    protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
                                            EventExecutorChooserFactory chooserFactory, Object... args) {
        if (nThreads <= 0) {
            throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
        }

        if (executor == null) {// Tony: 如果执行器为空，则创建一个
          
            // EventLoop都是通过executor创建线程并执行它的
            executor = new ThreadPerTaskExecutor(newDefaultThreadFactory()); // 线程创建器，源码见下面
        }
        // EventLoop是EventExecutor接口的具体实现
        children = new EventExecutor[nThreads];

        for (int i = 0; i < nThreads; i ++) {
            boolean success = false;
            try {
                // 有多个实现方法，见下面 返回NioEventLoop
                children[i] = newChild(executor, args);
                success = true;
            } catch (Exception e) {
                // TODO: Think about if this is a good exception type
                throw new IllegalStateException("failed to create a child event loop", e);
            } finally {
                if (!success) {
                    for (int j = 0; j < i; j ++) {
                        children[j].shutdownGracefully();
                    }

                    for (int j = 0; j < i; j ++) {
                        EventExecutor e = children[j];
                        try {
                            while (!e.isTerminated()) {
                                e.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
                            }
                        } catch (InterruptedException interrupted) {
                            // Let the caller handle the interruption.
                            Thread.currentThread().interrupt();
                            break;
                        }
                    }
                }
            }
        }

        chooser = chooserFactory.newChooser(children);

        final FutureListener<Object> terminationListener = new FutureListener<Object>() {
            @Override
            public void operationComplete(Future<Object> future) throws Exception {
                if (terminatedChildren.incrementAndGet() == children.length) {
                    terminationFuture.setSuccess(null);
                }
            }
        };

        for (EventExecutor e: children) {
            e.terminationFuture().addListener(terminationListener);
        }

        Set<EventExecutor> childrenSet = new LinkedHashSet<EventExecutor>(children.length);
        Collections.addAll(childrenSet, children);
        readonlyChildren = Collections.unmodifiableSet(childrenSet);
    }

ThreadPerTaskExecutor创建线程

public final class ThreadPerTaskExecutor implements Executor {
    private final ThreadFactory threadFactory;

    public ThreadPerTaskExecutor(ThreadFactory threadFactory) {
        if (threadFactory == null) {
            throw new NullPointerException("threadFactory");
        }
        this.threadFactory = threadFactory;
    }

    @Override
    public void execute(Runnable command) {
        threadFactory.newThread(command).start();
    }
}

NioEventLoopGroup

public class NioEventLoopGroup extends MultithreadEventLoopGroup {
  // 省略代码

    @Override
    protected EventLoop newChild(Executor executor, Object... args) throws Exception {
        return new NioEventLoop(this, executor, (SelectorProvider) args[0],
            ((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
    }
}

返回NioEventLoop，也传入了executor，用来帮助创建线程执行任务

看NioEventLoop的具体实现

public final class NioEventLoop extends SingleThreadEventLoop {
    // 代码省略
    NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
                 SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
        super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
        if (selectorProvider == null) {
            throw new NullPointerException("selectorProvider");
        }
        if (strategy == null) {
            throw new NullPointerException("selectStrategy");
        }
        provider = selectorProvider;
        final SelectorTuple selectorTuple = openSelector();
        selector = selectorTuple.selector;
        unwrappedSelector = selectorTuple.unwrappedSelector;
        selectStrategy = strategy;
    }
    // 代码省略

selector是NIO的selector

 NioEventLoop将通道注册到EventLoop的selector上，进行事件轮询

不断追踪NioEventLoop

最顶层是

public interface Executor {

    /**
     * Executes the given command at some time in the future.  The command
     * may execute in a new thread, in a pooled thread, or in the calling
     * thread, at the discretion of the {@code Executor} implementation.
     *
     * @param command the runnable task
     * @throws RejectedExecutionException if this task cannot be
     * accepted for execution
     * @throws NullPointerException if command is null
     */
    void execute(Runnable command);
}

execute由SingleThreadEventExecutor实现

提交任务

 public abstract class SingleThreadEventExecutor extends AbstractScheduledEventExecutor implements OrderedEventExecutor {
    // 省略代码  
    @Override
    public void execute(Runnable task) {
        if (task == null) {
            throw new NullPointerException("task");
        }
        //  判断execute方法的调用者是不是EventLoop同一个线程
        boolean inEventLoop = inEventLoop();
        addTask(task);// 增加到任务队列
        if (!inEventLoop) {// 不是同一个线程，则调用启动方法
            startThread();
            if (isShutdown()) {
                boolean reject = false;
                try {
                    if (removeTask(task)) {
                        reject = true;
                    }
                } catch (UnsupportedOperationException e) {
                    // The task queue does not support removal so the best thing we can do is to just move on and
                    // hope we will be able to pick-up the task before its completely terminated.
                    // In worst case we will log on termination.
                }
                if (reject) {
                    reject();
                }
            }
        }

        if (!addTaskWakesUp && wakesUpForTask(task)) {
            wakeup(inEventLoop);
        }
    }

startThread

    private void startThread() {
        if (state == ST_NOT_STARTED) {
            if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
                try {
                    doStartThread();// Tony: 未启动，则触发启动
                } catch (Throwable cause) {
                    STATE_UPDATER.set(this, ST_NOT_STARTED);
                    PlatformDependent.throwException(cause);
                }
            }
        }
    }

    private void doStartThread() {
        assert thread == null;
        executor.execute(new Runnable() {// 这里的executor是初始化EventLoop的时候传进来的
            @Override
            public void run() {
                thread = Thread.currentThread();
                if (interrupted) {
                    thread.interrupt();
                }

                boolean success = false;
                updateLastExecutionTime();
                try {// 创建线程开始执行run方法，所以，每个EventLoop都是执行run
                    SingleThreadEventExecutor.this.run();
                    success = true;
                } catch (Throwable t) {
                    logger.warn("Unexpected exception from an event executor: ", t);
                } finally {
                    for (;;) {
                        int oldState = state;
                        if (oldState >= ST_SHUTTING_DOWN || STATE_UPDATER.compareAndSet(
                                SingleThreadEventExecutor.this, oldState, ST_SHUTTING_DOWN)) {
                            break;
                        }
                    }

                    // Check if confirmShutdown() was called at the end of the loop.
                    if (success && gracefulShutdownStartTime == 0) {
                        if (logger.isErrorEnabled()) {
                            logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " +
                                    SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must " +
                                    "be called before run() implementation terminates.");
                        }
                    }

                    try {
                        // Run all remaining tasks and shutdown hooks.
                        for (;;) {
                            if (confirmShutdown()) {
                                break;
                            }
                        }
                    } finally {
                        try {
                            cleanup();
                        } finally {
                            STATE_UPDATER.set(SingleThreadEventExecutor.this, ST_TERMINATED);
                            threadLock.release();
                            if (!taskQueue.isEmpty()) {
                                if (logger.isWarnEnabled()) {
                                    logger.warn("An event executor terminated with " +
                                            "non-empty task queue (" + taskQueue.size() + ')');
                                }
                            }

                            terminationFuture.setSuccess(null);
                        }
                    }
                }
            }
        });
    }

@Override
    protected void run() {// 有任务提交后，被触发执行
        for (;;) {// 执行两件事selector,select的事件 和 taskQueue里面的内容
            try {
                try {
                    switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                    case SelectStrategy.CONTINUE:
                        continue;

                    case SelectStrategy.BUSY_WAIT:
                        // fall-through to SELECT since the busy-wait is not supported with NIO

                    case SelectStrategy.SELECT:
                        select(wakenUp.getAndSet(false));

                        // 'wakenUp.compareAndSet(false, true)' is always evaluated
                        // before calling 'selector.wakeup()' to reduce the wake-up
                        // overhead. (Selector.wakeup() is an expensive operation.)
                        //
                        // However, there is a race condition in this approach.
                        // The race condition is triggered when 'wakenUp' is set to
                        // true too early.
                        //
                        // 'wakenUp' is set to true too early if:
                        // 1) Selector is waken up between 'wakenUp.set(false)' and
                        //    'selector.select(...)'. (BAD)
                        // 2) Selector is waken up between 'selector.select(...)' and
                        //    'if (wakenUp.get()) { ... }'. (OK)
                        //
                        // In the first case, 'wakenUp' is set to true and the
                        // following 'selector.select(...)' will wake up immediately.
                        // Until 'wakenUp' is set to false again in the next round,
                        // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
                        // any attempt to wake up the Selector will fail, too, causing
                        // the following 'selector.select(...)' call to block
                        // unnecessarily.
                        //
                        // To fix this problem, we wake up the selector again if wakenUp
                        // is true immediately after selector.select(...).
                        // It is inefficient in that it wakes up the selector for both
                        // the first case (BAD - wake-up required) and the second case
                        // (OK - no wake-up required).

                        if (wakenUp.get()) {
                            selector.wakeup();
                        }
                        // fall through
                    default:
                    }
                } catch (IOException e) {
                    // If we receive an IOException here its because the Selector is messed up. Let's rebuild
                    // the selector and retry. https://github.com/netty/netty/issues/8566
                    rebuildSelector0();
                    handleLoopException(e);
                    continue;
                }

                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;
                if (ioRatio == 100) {
                    try {// 处理事件
                        processSelectedKeys();
                    } finally {
                        // Ensure we always run tasks.
                        runAllTasks();
                    }
                } else {
                    final long ioStartTime = System.nanoTime();
                    try {
                        processSelectedKeys();
                    } finally {
                        // Ensure we always run tasks.
                        final long ioTime = System.nanoTime() - ioStartTime;
                        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
            // Always handle shutdown even if the loop processing threw an exception.
            try {
                if (isShuttingDown()) {
                    closeAll();
                    if (confirmShutdown()) {
                        return;
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
        }
    }

    private static void handleLoopException(Throwable t) {
        logger.warn("Unexpected exception in the selector loop.", t);

        // Prevent possible consecutive immediate failures that lead to
        // excessive CPU consumption.
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // Ignore.
        }
    }

    private void processSelectedKeys() {
        if (selectedKeys != null) {
            processSelectedKeysOptimized();
        } else {// 处理事件
            processSelectedKeysPlain(selector.selectedKeys());
        }
    }

private void processSelectedKeysPlain(Set<SelectionKey> selectedKeys) {
        // check if the set is empty and if so just return to not create garbage by
        // creating a new Iterator every time even if there is nothing to process.
        // See https://github.com/netty/netty/issues/597
        if (selectedKeys.isEmpty()) {
            return;
        }
        // 获取selector所有选中的事件（ServerSocketChannel主要是OP_ACCEPT,SocketChannle主要是OP_READ）
        Iterator<SelectionKey> i = selectedKeys.iterator();
        for (;;) {
            final SelectionKey k = i.next();
            final Object a = k.attachment();
            i.remove();

            if (a instanceof AbstractNioChannel) {//  处理niochannel事件
                processSelectedKey(k, (AbstractNioChannel) a);
            } else {
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }

            if (!i.hasNext()) {
                break;
            }

            if (needsToSelectAgain) {
                selectAgain();
                selectedKeys = selector.selectedKeys();

                // Create the iterator again to avoid ConcurrentModificationException
                if (selectedKeys.isEmpty()) {
                    break;
                } else {
                    i = selectedKeys.iterator();
                }
            }
        }
    }

EventLoop自身实现了Executor接口，当调用executor方法提交任务时，则判断是否启动，未启动则调用内置的executor创建新线程来触发run方法执行

channel注册到selector上

请求

服务端启动的过程，服务端的启动就是Bind绑定端口的过程

回到EchoServer

追踪bind源码

 // Start the server.
ChannelFuture f = b.bind(PORT).sync();

bind绑定端口并创建通道

 public ChannelFuture bind(int inetPort) {
        return bind(new InetSocketAddress(inetPort)); // 绑定端口的入口代码
    }

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(String inetHost, int inetPort) {
        return bind(SocketUtils.socketAddress(inetHost, inetPort));
    }

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(InetAddress inetHost, int inetPort) {
        return bind(new InetSocketAddress(inetHost, inetPort));
    }

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(SocketAddress localAddress) {
        validate();
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        return doBind(localAddress);// 真正干事的代码
    }

    private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();// 创建/初始化ServerSocketChannel对象，并注册到Selector
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }
        // 等注册完成之后，再绑定端口。 防止端口开放了，却不能处理请求
        if (regFuture.isDone()) {
            // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();
            doBind0(regFuture, channel, localAddress, promise);// 实际操作绑定端口的代码
            return promise;
        } else {
            // Registration future is almost always fulfilled already, but just in case it's not.
            final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    Throwable cause = future.cause();
                    if (cause != null) {
                        // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
                        // IllegalStateException once we try to access the EventLoop of the Channel.
                        promise.setFailure(cause);
                    } else {
                        // Registration was successful, so set the correct executor to use.
                        // See https://github.com/netty/netty/issues/2586
                        promise.registered();

                        doBind0(regFuture, channel, localAddress, promise);
                    }
                }
            });
            return promise;
        }
    }

    final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            channel = channelFactory.newChannel(); // 通道
            init(channel); // 初始化通道
        } catch (Throwable t) {
            if (channel != null) {
                // channel can be null if newChannel crashed (eg SocketException("too many open files"))
                channel.unsafe().closeForcibly();
                // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
                return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
            }
            // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
            return new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE).setFailure(t);
        }
        //（一开始初始化的group）MultithreadEventLoopGroup里面选择一个eventLoop进行绑定
        ChannelFuture regFuture = config().group().register(channel); // register见下面
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

        // If we are here and the promise is not failed, it's one of the following cases:
        // 1) If we attempted registration from the event loop, the registration has been completed at this point.
        //    i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
        // 2) If we attempted registration from the other thread, the registration request has been successfully
        //    added to the event loop's task queue for later execution.
        //    i.e. It's safe to attempt bind() or connect() now:
        //         because bind() or connect() will be executed *after* the scheduled registration task is executed
        //         because register(), bind(), and connect() are all bound to the same thread.

        return regFuture;
    }

    abstract void init(Channel channel) throws Exception;

    private static void doBind0(
            final ChannelFuture regFuture, final Channel channel,
            final SocketAddress localAddress, final ChannelPromise promise) {

        // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
        // the pipeline in its channelRegistered() implementation.
        channel.eventLoop().execute(new Runnable() {
            @Override
            public void run() {//这里向EventLoop提交任务，一旦有任务提交则会触发EventLoop的轮询
                if (regFuture.isSuccess()) {// 本质又绕回到channel的bind方法上面。
                    channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
                } else {
                    promise.setFailure(regFuture.cause());
                }
            }
        });
    }

@Override
public ChannelFuture register(Channel channel){
    return next().register(channel); // 根据选择器，选择一个合适的NioEventLoop进行注册(SingleEventLoop)
}

追踪register代码...

netty中的Channel是一个抽象的概念，可以理解为对JDK NIO Channel的增强和拓展。增加了很多属性和方法，下面罗列几个常见的属性和方法：

三、责任链设计模式

 责任链（Chain of Responsibility Pattern）为请求创建了一个处理对象的链。

发起请求和具体处理请求的过程进行解耦：职责链上的处理者负责处理请求，客户只需要将请求发送到职责链上即可，无须关心请求的处理细节和请求的传递。

 handler是具体处理请求的程序

 实现责任链模式4个要素：处理器抽象类、具体的处理器实现类、保存处理器信息、处理执行

 

 责任链代码示例

// -----链表形式调用------netty就是类似的这种形式
public class PipelineDemo {
    /**
     * 初始化的时候造一个head，作为责任链的开始，但是并没有具体的处理
     */
    public HandlerChainContext head = new HandlerChainContext(new AbstractHandler() {
        @Override
        void doHandler(HandlerChainContext handlerChainContext, Object arg0) {
            handlerChainContext.runNext(arg0);
        }
    });

    public void requestProcess(Object arg0) {
        this.head.handler(arg0);
    }

    public void addLast(AbstractHandler handler) {
        HandlerChainContext context = head;
        while (context.next != null) {
            context = context.next;
        }
        context.next = new HandlerChainContext(handler);
    }


    public static void main(String[] args) {
        PipelineDemo pipelineChainDemo = new PipelineDemo();
        pipelineChainDemo.addLast(new Handler2());
        pipelineChainDemo.addLast(new Handler1());
        pipelineChainDemo.addLast(new Handler1());
        pipelineChainDemo.addLast(new Handler2());

        // 发起请求
        pipelineChainDemo.requestProcess("火车呜呜呜~~");

    }
}

/**
 * handler上下文，我主要负责维护链，和链的执行
 */
class HandlerChainContext {
    HandlerChainContext next; // 下一个节点
    AbstractHandler handler;

    public HandlerChainContext(AbstractHandler handler) {
        this.handler = handler;
    }

    void handler(Object arg0) {
        this.handler.doHandler(this, arg0);
    }

    /**
     * 继续执行下一个
     */
    void runNext(Object arg0) {
        if (this.next != null) {
            this.next.handler(arg0);
        }
    }
}

// 处理器抽象类
abstract class AbstractHandler {
    /**
     * 处理器，这个处理器就做一件事情，在传入的字符串中增加一个尾巴..
     */
    abstract void doHandler(HandlerChainContext handlerChainContext, Object arg0); // handler方法
}

// 处理器具体实现类
class Handler1 extends AbstractHandler {
    @Override
    void doHandler(HandlerChainContext handlerChainContext, Object arg0) {
        arg0 = arg0.toString() + "..handler1的小尾巴.....";
        System.out.println("我是Handler1的实例，我在处理：" + arg0);
        // 继续执行下一个
        handlerChainContext.runNext(arg0);
    }
}

// 处理器具体实现类
class Handler2 extends AbstractHandler {
    @Override
    void doHandler(HandlerChainContext handlerChainContext, Object arg0) {
        arg0 = arg0.toString() + "..handler2的小尾巴.....";
        System.out.println("我是Handler2的实例，我在处理：" + arg0);
        // 继续执行下一个
        handlerChainContext.runNext(arg0);
    }
}

 Netty中的ChannelPipeline责任链

 Nettty中定义了很多事件

 Pipeline中的handler是什么？

 ChannelPipeline是线程安全的，ChannelHandler可以在任何时候添加或删除。

例如，可以在即将交换敏感信息时插入加密处理程序，并在交换后删除。

一般操作，初始化的时候增加进去，较少删除。下面是Pipeline中管理handler的API：

 源码查看

handler执行分析

分析registered入站事件的处理

 源码查看 从bind()进入

 

bind出站事件分析

 源码查看

 

分析accept入站事件的处理

 源码查看

 

read入站事件的处理

 源码查看

四、零拷贝机制

JDK ByteBuffer存在一些缺点：

• 无法动态扩容。长度是固定的，不能动态扩展和收缩，当数据大于ByteBuffer容量时，会发生索引越界异常。
• API使用复杂。读写的时候需要手工调用flip()和rewind()等方法，使用时需要非常谨慎的使用这些api，否则容易出错。

1.Netty自己的ByteBuf

ByteBuf是为解决ByteBuffer的问题和满足网络应用程序开发人员的日常需求而设计的。

ByteBuf的增强：

•  API操作便捷性
• 动态扩容
• 多种ByteBuf实现
• 高效的零拷贝机制

 2.ByteBuf操作

ByteBuf三个重要属性：capacity容量、readerIndex读取位置、writeIndex写入位置。

提供了两个指针变量来支持顺序读和写操作，分别时readerIndex和写操作writeIndex

常用方法定义：

   

 

 

下图显示了一个缓冲区是如何被两个指针分割成三个区域的：

示例代码

/**
 * bytebuf的常规API操作示例
 */
public class ByteBufDemo {
    @Test
    public void apiTest() {
        //  +-------------------+------------------+------------------+
        //  | discardable bytes |  readable bytes  |  writable bytes  |
        //  |                   |     (CONTENT)    |                  |
        //  +-------------------+------------------+------------------+
        //  |                   |                  |                  |
        //  0      <=       readerIndex   <=   writerIndex    <=    capacity

        // 1.创建一个非池化的ByteBuf，大小为10个字节
        ByteBuf buf = Unpooled.buffer(10);
        System.out.println("原始ByteBuf为====================>" + buf.toString());
        System.out.println("1.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 2.写入一段内容
        byte[] bytes = {1, 2, 3, 4, 5};
        buf.writeBytes(bytes);
        System.out.println("写入的bytes为====================>" + Arrays.toString(bytes));
        System.out.println("写入一段内容后ByteBuf为===========>" + buf.toString());
        System.out.println("2.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 3.读取一段内容
        byte b1 = buf.readByte();
        byte b2 = buf.readByte();
        System.out.println("读取的bytes为====================>" + Arrays.toString(new byte[]{b1, b2}));
        System.out.println("读取一段内容后ByteBuf为===========>" + buf.toString());
        System.out.println("3.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 4.将读取的内容丢弃
        buf.discardReadBytes();
        System.out.println("将读取的内容丢弃后ByteBuf为========>" + buf.toString());
        System.out.println("4.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 5.清空读写指针
        buf.clear();
        System.out.println("将读写指针清空后ByteBuf为==========>" + buf.toString());
        System.out.println("5.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 6.再次写入一段内容，比第一段内容少
        byte[] bytes2 = {1, 2, 3};
        buf.writeBytes(bytes2);
        System.out.println("写入的bytes为====================>" + Arrays.toString(bytes2));
        System.out.println("写入一段内容后ByteBuf为===========>" + buf.toString());
        System.out.println("6.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 7.将ByteBuf清零
        buf.setZero(0, buf.capacity());
        System.out.println("将内容清零后ByteBuf为==============>" + buf.toString());
        System.out.println("7.ByteBuf中的内容为================>" + Arrays.toString(buf.array()) + "
");

        // 8.再次写入一段超过容量的内容
        byte[] bytes3 = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};
        buf.writeBytes(bytes3);
        System.out.println("写入的bytes为====================>" + Arrays.toString(bytes3));
        System.out.println("写入一段内容后ByteBuf为===========>" + buf.toString());
        System.out.println("8.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");
        //  随机访问索引 getByte
        //  顺序读 read*
        //  顺序写 write*
        //  清除已读内容 discardReadBytes
        //  清除缓冲区 clear
        //  搜索操作
        //  标记和重置
        //  完整代码示例：参考
        // 搜索操作 读取指定位置 buf.getByte(1);
        //
    }

}

Unpooled推荐的方式创建buf

可以动态扩容

运行结果

原始ByteBuf为====================>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 0, cap: 10)
1.ByteBuf中的内容为===============>[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

写入的bytes为====================>[1, 2, 3, 4, 5]
写入一段内容后ByteBuf为===========>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 5, cap: 10)
2.ByteBuf中的内容为===============>[1, 2, 3, 4, 5, 0, 0, 0, 0, 0]

读取的bytes为====================>[1, 2]
读取一段内容后ByteBuf为===========>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 2, widx: 5, cap: 10)
3.ByteBuf中的内容为===============>[1, 2, 3, 4, 5, 0, 0, 0, 0, 0]

将读取的内容丢弃后ByteBuf为========>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 3, cap: 10)
4.ByteBuf中的内容为===============>[3, 4, 5, 4, 5, 0, 0, 0, 0, 0]

将读写指针清空后ByteBuf为==========>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 0, cap: 10)
5.ByteBuf中的内容为===============>[3, 4, 5, 4, 5, 0, 0, 0, 0, 0]

写入的bytes为====================>[1, 2, 3]
写入一段内容后ByteBuf为===========>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 3, cap: 10)
6.ByteBuf中的内容为===============>[1, 2, 3, 4, 5, 0, 0, 0, 0, 0]

将内容清零后ByteBuf为==============>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 3, cap: 10)
7.ByteBuf中的内容为================>[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

写入的bytes为====================>[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
写入一段内容后ByteBuf为===========>UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 14, cap: 64)
8.ByteBuf中的内容为===============>[0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

动态扩容

io.netty.buffer.AbstractByteBuf下：

  final void ensureWritable0(int minWritableBytes) {
        ensureAccessible();
        if (minWritableBytes <= writableBytes()) {
            return;
        }
        final int writerIndex = writerIndex();
        if (checkBounds) {
            if (minWritableBytes > maxCapacity - writerIndex) {
                throw new IndexOutOfBoundsException(String.format(
                        "writerIndex(%d) + minWritableBytes(%d) exceeds maxCapacity(%d): %s",
                        writerIndex, minWritableBytes, maxCapacity, this));
            }
        }

        // Normalize the current capacity to the power of 2.
        int minNewCapacity = writerIndex + minWritableBytes;
        int newCapacity = alloc().calculateNewCapacity(minNewCapacity, maxCapacity);

        int fastCapacity = writerIndex + maxFastWritableBytes();
        // Grow by a smaller amount if it will avoid reallocation
        if (newCapacity > fastCapacity && minNewCapacity <= fastCapacity) {
            newCapacity = fastCapacity;
        }

        // Adjust to the new capacity.
        capacity(newCapacity);
    }

calculateNewCapacity

    @Override
    public int calculateNewCapacity(int minNewCapacity, int maxCapacity) { // minNewCapacity：14 maxCapacity：2147483647
        checkPositiveOrZero(minNewCapacity, "minNewCapacity");
        if (minNewCapacity > maxCapacity) { // minCapacity：14
            throw new IllegalArgumentException(String.format(
                    "minNewCapacity: %d (expected: not greater than maxCapacity(%d)",
                    minNewCapacity, maxCapacity));
        } // 阈值4M，这个阈值的用意：容量要求4M以内，每次扩容以2的倍数进行计算，超过4M容量，另外的计算方式。
        final int threshold = CALCULATE_THRESHOLD; // 4 MiB page

        if (minNewCapacity == threshold) { // 新容量的最小要求，如果等于阈值，则立刻返回
            return threshold;
        }

        // If over threshold, do not double but just increase by threshold.
        if (minNewCapacity > threshold) {
            int newCapacity = minNewCapacity / threshold * threshold;
            if (newCapacity > maxCapacity - threshold) {
                newCapacity = maxCapacity;
            } else {
                newCapacity += threshold;
            }
            return newCapacity;
        }
        // 如果容量要求没超过阈值，则从64字节开始，不断增加一倍，直至满足新容量最小要求
        // Not over threshold. Double up to 4 MiB, starting from 64.
        int newCapacity = 64;
        while (newCapacity < minNewCapacity) {
            newCapacity <<= 1;
        }

        return Math.min(newCapacity, maxCapacity);
    }

选择合适的ByteBuf实现：

了解核心的3个维度的划分方式，8种具体实现

 在使用中，都是使用ByteBufAllocator分配器进行申请，同时分配器具有内存管理的功能

堆外内存示例

/**
 * 堆外内存的常规API操作示例
 */
public class DirectByteBufDemo {
    @Test
    public void apiTest() {
        //  +-------------------+------------------+------------------+
        //  | discardable bytes |  readable bytes  |  writable bytes  |
        //  |                   |     (CONTENT)    |                  |
        //  +-------------------+------------------+------------------+
        //  |                   |                  |                  |
        //  0      <=       readerIndex   <=   writerIndex    <=    capacity

        // 1.创建一个非池化的ByteBuf，大小为10个字节
        ByteBuf buf = Unpooled.directBuffer(10);
        System.out.println("原始ByteBuf为====================>" + buf.toString());
        // System.out.println("1.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
"); 堆外内存不能用buf.array()

        // 2.写入一段内容
        byte[] bytes = {1, 2, 3, 4, 5};
        buf.writeBytes(bytes);
        System.out.println("写入的bytes为====================>" + Arrays.toString(bytes));
        System.out.println("写入一段内容后ByteBuf为===========>" + buf.toString());
        //System.out.println("2.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 3.读取一段内容
        byte b1 = buf.readByte();
        byte b2 = buf.readByte();
        System.out.println("读取的bytes为====================>" + Arrays.toString(new byte[]{b1, b2}));
        System.out.println("读取一段内容后ByteBuf为===========>" + buf.toString());
       //System.out.println("3.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 4.将读取的内容丢弃
        buf.discardReadBytes();
        System.out.println("将读取的内容丢弃后ByteBuf为========>" + buf.toString());
        //System.out.println("4.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 5.清空读写指针
        buf.clear();
        System.out.println("将读写指针清空后ByteBuf为==========>" + buf.toString());
        //System.out.println("5.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 6.再次写入一段内容，比第一段内容少
        byte[] bytes2 = {1, 2, 3};
        buf.writeBytes(bytes2);
        System.out.println("写入的bytes为====================>" + Arrays.toString(bytes2));
        System.out.println("写入一段内容后ByteBuf为===========>" + buf.toString());
       // System.out.println("6.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");

        // 7.将ByteBuf清零
        buf.setZero(0, buf.capacity());
        System.out.println("将内容清零后ByteBuf为==============>" + buf.toString());
       // System.out.println("7.ByteBuf中的内容为================>" + Arrays.toString(buf.array()) + "
");

        // 8.再次写入一段超过容量的内容
        byte[] bytes3 = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};
        buf.writeBytes(bytes3);
        System.out.println("写入的bytes为====================>" + Arrays.toString(bytes3));
        System.out.println("写入一段内容后ByteBuf为===========>" + buf.toString());
       // System.out.println("8.ByteBuf中的内容为===============>" + Arrays.toString(buf.array()) + "
");
        //  随机访问索引 getByte
        //  顺序读 read*
        //  顺序写 write*
        //  清除已读内容 discardReadBytes
        //  清除缓冲区 clear
        //  搜索操作
        //  标记和重置
        //  完整代码示例：参考
        // 搜索操作 读取指定位置 buf.getByte(1);
        //
    }

}

Unsafe的实现

 

内存复用

 

PooledByteBuf对象、内存复用

3.零拷贝机制

Netty的零拷贝机制，是一种应用层的实现。和底层的JVM、操作系统内存机制并无过多关联。

 代码示例

/**
 * 零拷贝示例
 */
public class ZeroCopyTest {
    @org.junit.Test
    public void wrapTest() {
        byte[] arr = {1, 2, 3, 4, 5};
        ByteBuf byteBuf = Unpooled.wrappedBuffer(arr);
        System.out.println(byteBuf.getByte(4));
        arr[4] = 6;
        System.out.println(byteBuf.getByte(4));
    } // java数组转为buf 5 arr修改为6后，byteBuf也变为6，说明两者用的是相同的数据，零拷贝

    @org.junit.Test
    public void sliceTest() {
        ByteBuf buffer1 = Unpooled.wrappedBuffer("hello".getBytes());
        ByteBuf newBuffer = buffer1.slice(1, 2);
        newBuffer.unwrap();
        System.out.println(newBuffer.toString());
    } // 拆分。不会动原来的buf，还保留原来buf的地址

    @org.junit.Test
    public void compositeTest() {
        ByteBuf buffer1 = Unpooled.buffer(3);
        buffer1.writeByte(1);
        ByteBuf buffer2 = Unpooled.buffer(3);
        buffer2.writeByte(4);
        CompositeByteBuf compositeByteBuf = Unpooled.compositeBuffer();
        CompositeByteBuf newBuffer = compositeByteBuf.addComponents(true, buffer1, buffer2);
        System.out.println(newBuffer);
    } // 合并。还保留原来buf的信息

}